1. Field of the Invention
The present invention relates to a dispersing and grinding apparatus in which grinding medium particulates, such as balls or beads, are agitated together with a material to be ground to apply shearing forces to the material, thereby finely grinding the material and dispersing it in a liquid.
2. Background Information
Various apparatuses are known and used for dispersing a material by means of a grinding medium. One such apparatus that is widely used has disks or rods protruding on an agitating shaft disposed within a grinding vessel to induce flow of the grinding medium. Another such apparatus is an annular-type dispersing and grinding apparatus, wherein a cylinder is disposed within the grinding vessel, and a narrow annular flow path is defined between the cylinder and the inner wall of the grinding vessel, thereby inducing flow of the grinding medium within the annular flow path.
In the annular-type dispersing and grinding apparatus, the material to be ground is fed from an inlet into the grinding vessel by a feeding means, such as a pump, dispersed within the annular flow path, and then discharged through an outlet. The mixture of the material and grinding medium flows through the annular flow path, from the inlet side toward the outlet side, and since only the dispersed material is discharged, the grinding medium tends to gather near the outlet side. For this reason, the grinding medium becomes unevenly distributed throughout the grinding vessel, and thus the dispersion efficiency becomes poor.
In order to solve the problem of uneven distribution of the grinding medium in the conventional annular-type dispersing and grinding apparatus, the present inventors previously developed a dispersing and grinding apparatus in which a forward screw is provided on the outer surface of a cylindrical rotor for inducing the flow of the grinding medium from the inlet side to the outlet side, and a backward screw is provided on the inner wall surface of the rotor for inducing the flow of the grinding medium from the outlet side back to the inlet side. This type apparatus is described, for example, in U.S. Pat. No. 4,856,717. In this apparatus, the grinding medium continually circulates through the outside and the inside of the rotor, thereby reducing the uneven distribution of the grinding medium.
The present inventors also previously developed a dispersing and grinding apparatus in which a guiding means is formed on the outer peripheral surface of the rotor for guiding the flow of the mixture of the material and grinding medium in the circumferential direction. One such apparatus is described in U.S. Pat. No. 4,919,347. In this apparatus, the flow of the mixture approximates that of a plug flow within the grinding vessel, and the mixture receives sufficient dispersion treatment during its flow from the inlet side to the outlet side.
These dispersing and grinding apparatuses previously proposed by the present inventors can attain good dispersion efficiencies. However, notwithstanding the improved dispersion efficiency of the apparatus disclosed in U.S. Pat. No. 4,919,347, there is still a tendency for the grinding medium to gather near the outlet side. Further, in the dispersing and grinding apparatus as disclosed in U.S. Pat. No. 4,856,717, the uneven distribution of the medium can be markedly reduced, but the movement of the medium is slow as compared with the apparatus indicated in U.S. Pat. No. 4,919,347, so that the treatment time is sometimes very long depending on the composition of the material. In addition, in the annular-type dispersing and grinding apparatus, the grinding medium moves around the rotor, so that if the return inlet for the grinding medium is located at the center portion of the rotor, the grinding medium enters the inlet inefficiently and with great difficulty.